Optical cells for in-line spectroscopic analysis of chemical samples is important for the chemical process industry. In-line spectroscopic analysis enables the real-time determination of chemical content and concentration of chemical samples (i.e., qualitative and quantitative analysis) as the chemical is being generated or used in the chemical process.
Spectroscopic analyzers typically utilize optical cells, an apparatus for introducing a chemical sample into the cell, a light source shining on the cell, and data collection and analysis instrumentation. Optical cells are available for use with various light sources, sample types, sample introduction methods, and collection and analysis modules. Optical cells are typically used for off-line batch sample analysis, but in-line optical analysis cells having limited performance characteristics also exist. Optical cells are designed for gaseous and liquid sample analysis.
Optical cells are used to hold the sample adjacent to the optical source allowing analysis of the chemical sample. The prior art optical cells typically use windows comprising the light transmitting material sealed to a pressure resistant housing. This configuration of the prior art cells may leak, especially at high-pressures, soiling and/or damaging instruments, skewing test results, and/or necessitating time consuming cell cleanup.
U.S. Pat. No. 3,370,502 (Wilks), U.S. Pat. No. 4,595,833 (Sting), U.S. Pat. No. 4,988,195 (Doyle), U.S. Pat. No. 5,220,401 (Milosevic) disclose cylindrical rod-shaped multiple internal reflection elements having various end-shapes for improved optical transmission. They contemplate external sample chambers defined by a housing and the rod-shaped cylindrical optical transmitting members. For each of these inventions, the sample is introduced around the rod-shaped optical crystal. Wilks includes an adjustable housing for adjusting the sample area. For each of these inventions, the sample chamber is defined using multiple pieces that can allow the sample material to leak, especially at high pressures.
U.S. Pat. No. 4,614,428 (Harris), U.S. Pat. No. 5,003,174 (Datwyler), and U.S. Pat. No. 5,124,555 (HartI) disclose windows comprised of optical transmitting members. Further, this art teaches multiple piece sample chambers. In addition, Harris, Datwyler and HartI teach sample chambers defined by optical windows and the cell housing. Harris includes a two-piece cell and two optical transmitting members defining a sample chamber. Datwyler discloses an internal sample chamber defined by the cell and two optical windows inserted in opposing ends of the cell. HartI discloses an optical disc-shaped window having a flat circular measurement surfaced formed by rounding or cutting a conical shape into a flat optical disc material. The internal sample chamber in HartI is defined by using two rounded windows and the cell housing. Other such conically-shaped, window cell designs are known in the art and disclosed in Stromberg, H. D., Schock, R. N., "A Window Configuration for High Pressure Optical Cells", Review of Scientific Instruments, Vol. 41, No. 12 (December 1970).
Optical fiber light transmitting members used in cells are known in the art. Rice, Jeanette K., et al., in "New Fiber-Optic-Based High-Pressure Cell for Fluorescence Measurements in Near- and Supercritical Solvents," Applied Spectroscopy, Vol. 48, No. 8 (1994), pp. 1030-32, and Heglund et. al. In "Simple Fiber-Optic Interface for On-Line Supercritical Fluid Extraction Fourier Transform Infrared Spectrometry," Analytical Chemistry, Vol. 66, No. 20 (Oct. 15, 1994), pp. 3543-51 disclose a typical fiber optic configuration. In this configuration, a stainless steel cross-shaped cell and the optical fiber define the internal sample chamber. The optical fiber passes axially through one axis of the cross; the sample fluid passes through the opposing axes of the cross surrounding the optical transmitting member.
A disadvantage of the prior art is that sample fluids can leak from the cells. This occurs because it is difficult to achieve a leak-free seal between metal cell-housings and the optical transmitting materials used in the art. While in some cases fused seals are used for leak free operation, these applications are limited to either batch measurement or have the added disadvantage of having dead volume, and/or not being readily reusable due to the time and expense of cleaning. Thus, leak-free in-line high-pressure analysis is limited with the cells known in the art.
An additional disadvantage of the prior art is that cells known in the art comprise numerous parts, and are bulky and complex. These parts include at least two optical crystalline material windows, two-piece adjustable housings, and two-piece fused cells. This abundance of parts increases the possibility of leaking, especially at high pressures.
What is desired, therefore, is an optical cell for reliable use at high pressure in spectroscopic analyzers which is low-cost, relatively easy to assemble and clean, and minimizes dead zone.